A problem relating to catalytic converters for internal combustion engines, such as the prior art reversing flow catalytic converter for internal combustion engines disclosed in U.S. Pat. No. 6,148,613, is overheating. Lean burn combustion systems for fuel-efficient vehicles are particularly hard on exhaust after-treatment systems because excessive oxygen is always present in the exhaust. For example, the exhaust of diesel dual fuel (DDF) engines, which is one type of diesel engine, normally contains more than 5% volumetric oxygen after combustion. Under partial load the surplus of oxygen in the exhaust may be higher than 10% by volume. Under such circumstances, any engine management problems that result in excessive fuel in the exhaust, will generally damage exhaust after-treatment system due to overheating.
If a fuel management problem occurs, a large amount of the excess fuel delivered to the engine can pass through it and into the engine exhaust. That fuel will burn inside the catalyst if sufficient oxygen is available and the catalyst has reached catalytic temperature. For example, the complete burning of 2% of methane in the exhaust, can raise the temperature of exhaust gases by about 420° C., in addition to the 600° C. temperature of the exhaust as it is ejected from the engine. Consequently, the rate of temperature rise in the catalyst can reach 20 to 30° C./second, if the monoliths are metallic. Besides the catalytic burning of methane, any combustible matter such as soot accumulated on the catalyst surface, will also be rapidly oxidized under such high temperatures. The burning of accumulated soot will escalate and prolong the temperature rise. The thermal wave oscillation produced by the reverse flow process will also expedite the rise of the peak temperature of the catalyst substrate. Once the catalyst temperature reaches 1200° C., a metallic substrate will begin to soften and subsequently lose mechanical strength. Further temperature rise will cause collapse of the substrate and eventual melt-down will occur when it is heated to 1400-1450° C. A detrimental uncontrolled temperature rise can damage a catalyst in less than 20 seconds.
In the prior art, when a catalyst protection mode is required for a gasoline engine, an extremely rich fuel/air mixture is delivered to the engine. Since all oxygen is basically consumed inside the engine during the over-rich combustion process, the engine exhaust contains no oxygen. The large amount of excessive fuel from the engine pulls down the catalyst temperature. In this type of catalyst protection mode, however, the carbon monoxide content of the exhaust gas is undesirably very high.
However, for lean burn systems such as diesel or dual fuel engines, the excessive fuel will not cool down the catalyst temperature because of the presence of a high concentration of oxygen in the exhaust. Furthermore, lean burn systems cannot burn stoichiometric fuel/air mixtures because of knocking restrictions. For knock-free operation of a dual fuel engine, the original compression ratio of the baseline diesel engine requires the pre-mixed natural gas/air mixture to be generally leaner than λ=1.5.
As well, the concept of the reversing flow catalytic converter has been found to offer nearly continuous oxidation of exhaust components, mainly unburned hydrocarbons and carbon monoxide, when used after natural gas or dual fuel engines, in a 13 mode test cycle. For this reason, such a catalytic converter will likely not require supplementary heat added to the converter to maintain oxidation temperature. However, for a diesel engine there are fewer hydrocarbons and CO in the exhaust stream providing less fuel in the emissions. Engine fuel will need to be added to the exhaust stream during idle and low power operation of the engine in order to maintain an oxidation temperature sufficient to convert Co and hydrocarbons (including particulates), however, a considerably lesser amount of fuel than would be required by a conventional uni-directional oxidation catalyst. For this reason, addition of fuel can also result in overheating of the catalyst, if too much fuel is added.
U.S. Pat. No. 6,148,613 discloses a prior art reversing flow catalytic converter for internal combustion engines. Such device 10 includes a valve housing 14 which reversibly directs exhaust gases through a “U” shaped passage having a catalytic material therein. A valve disk 42 having two openings 48 therein rotates around a central axis, wherein in a first position of such rotatable valve disk 42 the exhaust gases enter the exhaust cavity from an exhaust pipe and pass through one of the openings in valve disk 42 into the “U” shaped passage. In the second position of the rotatable valve disk 42, the disk 42 and corresponding openings 48 therein are rotated 90° so that each opening 48 communicates with the same cavity within the valve housing 14, but a different one of the ports communicating with the U-shaped passage, so that gas flow through the u-shaped passage is thereby able to be reversed.
Disadvantageously, prior art devices such as the type disclosed in U.S. Pat. No. 6,148,613 lack a safeguard system to protect such reversing flow catalytic converter from overheating, as may arise under any one or more of the conditions explained above.
Further, there exists a need for a continuously oxidizing filter particulate trap for diesel engine exhausts.